1. Technical Field of the Invention
The present invention relates to corrugated coaxial cables.
2. History of Related Art
Historically, coaxial cables for transmission of RF signals have been available with either smooth wall or corrugated outer conductors. These two different constructions offer particular advantages to the end users. For the same physical cable size and density of the foam dielectric, a smooth wall outer conductor coax construction offers higher velocity of propagation and lower attenuation but inferior bending and handling characteristics when compared to an equivalent cable with a corrugated outer conductor. When good handling and bending characteristics are important, coaxial cables with corrugated outer conductors have usually been used. This mechanical improvement is achieved, however, by some degradation of important electrical performance characteristics. The corrugated outer conductor by virtue of its geometric shape increases the capacitance of the cable. This reduces the velocity of the transmitted signal, and also increases the attenuation in a cable of fixed size because of the reduction in the diameter of the inner conductor of the cable, which is needed to maintain the required characteristic impedance. Additionally, during the manufacturing process to create corrugations and proper physical fit, the foam dielectric is compressed somewhat more than for smooth wall outer designs, resulting in denser dielectric and creating a higher dielectric constant medium. Until now, these factors have combined to place a practical limit on the velocity of a corrugated foam dielectric coaxial cable of rather less than 90%. The highest velocity in a commercially available cable of this type has been 89%.
Whether in a coaxial cable of smooth wall or corrugated outer conductor construction, achieving the highest practical velocity of signal propagation is advantageous, because this results in the lowest attenuation for a cable with fixed characteristic impedance and fixed size. The characteristic impedance is always set by system requirements, and is therefore fixed. The impedance of the cable has to be the same as that of the equipment items to which it is connected to minimize disrupting signal reflections. Wireless infrastructure systems typically use equipment with a 50 ohm characteristic impedance, while CATV (cable television) systems are usually 75 ohms. Cables are available in various sizes, larger sizes having lower attenuation than smaller sizes, and the lowest attenuation in a given size is advantageous because undesirable signal loss is minimized. In some cases the lower attenuation can allow a smaller cable to be used than would otherwise be possible, which is economically beneficial.
For a smooth wall cable, the relative propagation velocity (i.e., the velocity as a fraction of the velocity of light in air) is the reciprocal of the square root of the dielectric constant of the foam, and the dielectric constant is known for any particular foam density from equations available in the literature. To achieve a 90% propagation velocity for a smooth wall cable with a foamed polyethylene dielectric requires a foam density of approximately 0.22 g/cm3. In a corrugated cable, however, the electrical effect of the corrugations is to increase the capacitance of the cable and thus to decrease the velocity of propagation by a few percentage points. Corrugated cables that have been available for some years, and which have a velocity of propagation of 88% or 89% typically require a foam density of 0.18 g/cm3 or less, and consequently require a more advanced foam processing technology than do smooth wall cables, even with 90% or higher velocity. To view the difference another way, a smooth wall cable using a foam dielectric of the same density as has been used with corrugated cables for some years would have a velocity of 93% or greater.
In accordance with the present invention, there is provided a coaxial cable comprising an inner conductor, a foamed polymeric dielectric surrounding the inner conductor and having a foam density below 0.17 g/cm3, and a corrugated outer conductor surrounding the dielectric and dimensioned to provide the cable with a velocity of propagation greater than 90% of the speed of light, the corrugations in the outer conductor forming troughs and crests with the troughs engaging said dielectric.
The present invention provides a new design for corrugated cables which further improves the balance of electrical and mechanical characteristics attainable. Foam densities and corrugation dimensions are precisely controlled to realize a corrugated coaxial cable that retains the excellent flexibility and handling properties of corrugated cables and yet has a propagation velocity of 90% or greater, and with consequent improvement in attenuation.